More than a million small shareholders in National Grid Transco were on track for windfall payouts after the firm completed a major disposal programme.

The London-based group has finalised the £5.8 billion sale of four of its regional gas distribution networks, meaning it will return £2 billion to investors as previously announced.

Investors are due to receive a "B" share worth 65p for each share they hold. They will then choose whether to hold on to the stock, sell it for cash or swap it for a dividend.

National Grid has 1.4 million small shareholders, many of whom inherited the stock following the privatisation of regional electricity companies. The majority of these investors hold less than 500 shares. The payout is expected to be made around August, following an extraordinary general meeting.

The company said completion brought to an end a "long and complex" process.

Chief executive Roger Urwin said: "It marks the inauguration of what is in effect a new gas distribution industry for the UK."

Networks covering the North of England, Wales and the West, Scotland and the South of England were sold to various consortiums. The South of England and Scotland networks were bought by a consortium including Scottish & Southern Energy for £3.2 billion.

National Grid is still the UK's largest gas distributor as it has retained control of four other regional networks, including London and the North-west.